Study On Oxygen Defects Regulation,Structure And Ionic Conductivity In Scheelite BiVO₄-Based Materials

Solid ion conductor is the core component of solid oxide fuel cell,which is used to transport carriers and isolate effectively the reaction gas between cathode and anode.It plays a decisive role in the commercialization of solid oxide fuel cell.The existence of oxygen vacancies or interstitial oxygen defects is a prerequisite for oxygen migration in solid oxide conductors.Moreover,the presence of oxygen vacancies may introduce protons resulting in proton conduction.Therefore,understanding the stabilization of oxygen defects as well as the underlying mechanism of solid ionic conductors is key for developing more efficient solid ion conductors for solid oxide fuel cells.In this sense,compounds containing tetrahedral moieties have received growing attention as potential new solid ion conductors owing to the deformation and rotation flexibility of the tetrahedral units that may assist the stabilization of both vacancy defects or interstitial oxygens,as well as the transportation of carriers（both oxygen and proton ions）.The scheelite AMO₄-type compounds featured with isolated tetrahedral anions,is known as a class of potential solid ion conductor candidates due to its structural flexibility to transport interstitial oxygens,oxygen vacancies and even protons.However,In scheelite materials,electrolytes with oxygen vacancies exhibiting oxygen conduction are scarely reported,and the mechanisms for oxygen defect stabilization and oxide ion migration were not understood well.So far the scheelite dual-ion conductors including mixed oxygen ion and proton were not reported.In this study,The scheelite BiVO₄,a mixed oxygen and electron conductor,was investigated through an aliovalent substitution approach in Bi and V sites to create oxygen defects.The oxygen defects were regulated through adjusting the content of doping cations.A series of oxygen-defect-contained scheelite BiVO₄-based compositions were designed and prepared.The phase structures,structures stabilization,ionic conductivity as well as the conduction mechanism were investigated and characterized through a wide variety of experimental and computational methods.The main results are shown as following:1.In order to prepare scheelite-type ionic conductors with oxygen vacancies and understand the oxygen vacancy stabilization and migration,the oxygen-vacancy-contained BiVO₄ based solid solution were prepared through Sr²⁺substitution on Bi³⁺sites using conventional solid state method and the innovative aerodynamic levitation-laser heating（ADL）technique.The solid solution range of Bi_1-xSr_xVO_4-0.5xwas widened to x=0.3 and the single tetragonal scheelite phase were obtained for x=0.2 composition by the ADL technique.The conductivities of scheelite BiVO₄(¹0^-4 S/cm at 500℃)have been enhanced one order of magnitude(¹0^-3S/cm for Bi_0.95Sr_0.05VO_3.975 at 500℃)and the oxygen transport number was increased from 0.28 to 0.87 after substitution of Sr²⁺.The oxygen vacancies can be stabilized in Bi_1-xA_xVO_4-0.5x structure through Sr²⁺for Bi³⁺substitution,leading to corner-sharing V₂O₇ tetrahedral dimers,and migrate via a cooperative mechanism involving V₂O₇-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO₄ tetrahedra.2.In order to study the effect of doping cations size on phase formation,structure and ionic conductivity of scheelite BiVO₄,aliovalent substitution on Bi³⁺and V⁵⁺sites of BiVO₄were investigated to create oxygen vacancies or interstitial oxygen defects.The lowest oxygen defect formation energy（0.67 e V）,widest solid solution range（x≤0.15）and highest oxide ion conductivity(the conductivity and oxygen transport number is¹0^-3 S/cm and 0.92 for Bi_0.85Ca_0.15VO_3.925 sample at 500℃,respectively)was observed in Bi_1-xCa_xVO_4-0.5xmaterials,which resulted in the smaller lattice distortion and structure relaxation.3.Based on the oxygen vacancy-contained Bi_0.9Ca_0.1VO_3.95 composition,the effect of oxygen vacancy formation energy,structure and ionic conductivity evolution studies were extended to the double substituted Bi_0.9Ca_0.1V_1-yM_yO_3.95-d（M=Ti,Ge,P and Nb）compounds.This strategy was not able to form solid solutions in Bi_0.9Ca_0.1V_1-yTi_yO_3.95-0.5y and Bi_0.9Ca_0.1V_1-yP_yO_3.95 materials as they exhibit higher oxygen defect formation energy.The single tetragonal scheelite BiVO₄ phase were prepared in Ge⁴⁺and Nb⁵⁺doped Bi_0.9Ca_0.1VO_3.95compositions.The ionic conductivities decrease(⁵×10^-3 S/cm and³×10^-3S/cm before and after doping M at 700℃)and the activation energy increase gradually(0.62e V,0.73 e V and 0.83 e V for Bi_0.9Ca_0.1VO_3.95,Bi_0.9Ca_0.1V_0.9Ge_0.1O_3.9and Bi_0.9Ca_0.1V_0.75Nb_0.25O_3.95,respectively)in Bi_0.9Ca_0.1V_1-y My O_3.95-d（M=Ge and Nb）case with the decrease of vanadium.This indicates that the migration energy barrier of oxygen ion was increased and oxygen ion conductivities were reduced through changing the cationic type of tetrahedral central V⁵⁺ion in scheelite Bi_0.9Ca_0.1VO_3.95.4.In order to explore new scheelite mixed oxygen and proton ion conductors which exhibit low ohmic resistance without external gas humidification,the phases and impedance behaviour of the scheelite（1-x）BiVO₄-x La Nb O₄（x=0-1）based materials were investigated.Moreover,The solid solution of scheelite A_x(Bi_0.75La_0.25)_1-x(V_0.75Nb_0.25)O_4-0.5x（A=Ca,Sr,Ba）were prepared by doping with alkaline earth metal cations in（Bi,La）³⁺sites,suggesting that the solid solution range decreases upon doping with bigger dopants.The conductivities of scheelite Bi_0.75La_0.25V_0.75Nb_0.25O₄compound(²×10^-4 S/cm)have been enhanced 2.5 times after substitution of Ca²⁺and Sr²⁺(⁵×10^-4 S/cm)at 700℃.The study combining the complex impedance in dry and wet atmospheres demonstrate the ion conductivities in scheelite A_x(Bi_0.75La_0.25)_1-x(V_0.75Nb_0.25)O_4-0.5x（A=Ca,Sr）materials come from the joint contribution of both oxygen and proton ions.In this study,The oxygen-defect-contained scheelite BiVO₄-based materials were designed and prepared through cationic doping strategy.The oxygen vacancies were regulated by adjusting the content of the doping cations,and the defect stabilization and ionic conduction mechanism were studied using multiple complementary methods,as well as a series of scheelite BiVO₄-based solid ion conductors including oxygen vacancies conduction and mixed oxygen ion-proton conduction mechanism were obtained.It will provide electrolytes with potential application in solid state fuel cells and describe novel research methods for the study of defect structures and ion conduction mechanism.